Minimizing crazing of hot-dip aluminum coatings

ABSTRACT

The crazing resistance of hot dip Al coatings is substantially improved by adding from about 0.2 to 1.5 percent Mn to the molten Al bath.

United States Patent [19] Leonard [451 Oct. 15,1974

MlNlMlZlNG CRAZING 0F HOT-DIP ALUMINUM COATINGS [75] Inventor: Ralph W. Leonard, Plum Borough,

[73] Assignee: United States Steel Corporation, Pittsburgh, Pa. [22] Filed: Sept. 20, 1972 [21] Appl. No; 290,612

[52] US. Cl. 117/50, 117/114 C [51] Int. Cl. C23c 1/08 [58] Field of Search 117/51, 114 C, 50,114 R [56] References Cited UNITED STATES PATENTS 2,731,362 H1956 Brondyke ll7/5l 3,639,107 2/l972 Thompson ll7/l l4 C OTHER PUBLICATIONS Handbook of Chemistry and Physics, Chemical Rubber Publishing Co., 1949, pages 1975-1976.

Primary Examiner-Ralph S. Kendall Assistant Examiner.lacqueline Ware Attorney, Agent, or FirmArthur J. Greif [5 7 ABSTRACT The crazing resistance of hot dip Al coatings is substantially improved by adding from about 0.2 to 1.5 percent Mn t0 the molten Al bath.

5 Claims, 1 Drawing Figure 37 Bend /7' Bend 07' (Flat) Bend PAIENTEunm 151924 41394 37' Bend 7' Bend /F/af) Bend 1 MINlMlZING CRAZING OF HOT-DIP ALUMINUM COATINGS This invention relates to a hot-dipped, aluminized ferrous product with decreased tendency to crazing and to a method for producing am weight Mn to the aluminum-silicon bath will produce The art commonly employs two basically different a more. desirable g pe p f ffohsthueht molten baths for hot-dip aluminizing of ferrous sub- (w i F M are m comblnatlonl m h strates (sheet, strip, wire). For the production of bright final f hh The beneficial effect of Such Mh addl coated articles, the Si content is kept to minimum or hohs evidenced in the fohowlhg eXamPleS- residual values, generally below 0.25%. However, this Sh samples (26 gage) e coated n' am nuh type coated h hot eashy formed due to the silicon alloy bath compositions containing a variety of formahoh of a hrhhe hohaihhhhum alloy layer at the Mn and Fe concentrations. (See Table 1) Prior to coatmterface between the coatmg and ferrous substrate. ing the sampks were cleaned in i hl h l i kwhen formable: adherent coatings are q hf the led for four minutes in ten percent l-lCl solution, and generally p y from aholll 4 to l 1% the hath fluxed by immersion in a saturated aqueous solution of to retard the rate of hy g between the bath metal K2ZrF6. After fluxing, the samples were dried so as to and the ferrous substrate. The Of this is generleave a residue of powdered flu The samples emh) Presehtasadlscrete Phase the coahhg- {Although ployed in the bend tests were coated using identical significantly more adherent, these latter coatings nevdi i i e a bath temperature of l300F and an ertheless tend to craze, i.e., form fine cracks on the teni r i time of 60 seconds. Excess coating metal h Side formefl article Duflhg ubsequeht P9- and oxide residue were removed from the coated sheets sure. a d etr1mental iron-rust stam quickly develops in b use f an aipknift G i resistance was d t the f y of these h mined in controlled bend tests as described in ASTM It is therefore an ob ect of this invention to provide 525 71 Severity f Grazing was evaluated b ahlethofl for hllmmlllhg the crazlhg tendency of examination of the tension side at x magnification, (hp h wherein severity was rated as follows: 0--no crazing, Other ob ects and advantages of the mvention will be I crazing, 2 medium Grazing and 3 heavy h apparhht m a h g of the fohowmg f crazing. After a flat bend (0T), each sample was bent taken conluhchhh wlth the appended Claims 30 again sothat the next bend was over one sheet thickg zf m ness (1T). Samples were then bent over increasing hlustratlhh of a behdtest Spec" sheet thicknesses (see the FIGURE) until the number 9; t edeva of cram? tehdehcy' l d h of sheet thicknesses was reached in which no crazing m t 6 causes 0 crahmg revea e a was observed. Results are reported in Table l for both the S k propagatgd through masswe nehdle'shapeh (a) the number of sheet thicknesses over which sampartlcles 1n the coating. Electron-probe microanalysis plegwele bent to obtain no crazing, anddfol. (b) the of these i revehled a h contalhe-d bend condition. For each composition, the ratings reh and m "i always ported are the average of from four to eight different present in aluminum-coating baths. lt accumulates to samples v an equilibrium concentration, up to about three perw cent, by dissolution from the rigging and incoming 40 The beneficial effect provided by the purposeful adstrip.) It is believed that these particles are the brittle, dition of Mn (baths D through 0) can readily be seen intermetallic compound Fe Si Al The crystal strucfrom the above results. Thus, while additions within the ture of this compound is triclinic, and because of the range of from about 0.2 to 0.80% Mn provided the unsymmetrical nature of this lattice type, it grows as maximum resistance, even higher amounts of Mn proplatelets when it is the primary phase to solidify. These vided some enhancement in crazing resistance over the characteristics of Fe Si Al explain why the particles baths containing essentially no Mn (baths A through exhibit cleavage fracture during forming or bending op- "C).

u Mm TABLE l h Comparison of Crazing Tendency Number of Sheet Coating-Bath Bath" Thicknesses to Rating for Designation Composition \Vt.% Obtain No Crazing 3T Bend M K A 7.3 1.4 a 0.01 2.5 6,6 8.8 2.5 C (Ni 2.67 7.5 7.5 L5 D 0.23 2.5 7,6 3.4 0.4 E 0.37. 2.5 7.4 3,5 (7.5

F 0.54 0.90 7.7 2.2 ,0 G 0.52 1.71 7.6 3.8 0.4 H 0.48 2.37 7.6 4.1 0.8 1 0.63 1.22 7.7 3.0 1 0.4 .I 0.73 0.32 7.9 3.0 0 K 0.73 1.47 7.8 2.8 0.2 L 0.76 2.31 7.8 4.0 0.5 M 0.87 1.26 7.5 5.7 1.6 N 0.99 1.04 8.0 5.6 1.5 0 1.41 0.93 7.8 5.4 1.0

2 erations. It would therefore appear that the crack propagation tendency of these particles could be diminished if their morphology could be altered. it has now been found that the addition of from about 0.2 to about 1.5

'" Balance Aluminum The benefits of this invention may therefore be achieved by utilizing the following procedure. The cleaned strip is bright annealed under a controlled atmosphere, thereby obviating the need for fluxing. The annealed strip is cooled, preferably entering the coating bath at a temperature slightly above that of the bath. To prevent oxidation, the strip is maintained in a controlled atmosphere between the annealing furnace and the coating bath, thus permittingthe strip surface to alloy readily with the molten bath metal and form a continuous protective coating. The coated strip is then passed through an appropriate device, e.g. an air knife, for final control of the coating weight. Typical coating weights or thicknesses for sheet product vary from about 0.20 to about 1.0 ounces per square foot of sheet (i.e., about 0.4 to 2.0 mils thick per side).

The coating bath generally contains from about 7 to about 9 percent silicon and is maintained at temperatures ranging from about 1,180"! to about l,300F. A preferred practice employs a Mn content of about 0.4 to 0.8 percent, in a bath maintained at a temperature of l,200l ,250F, wherein iron content thereof will be less than about 1.5 percent. It has been found that manganese contents within the above preferred range reduces the maximum iron solubility from about 2.5 percent (manganese-free bath) to less than about 1.5 percent. Thus, the addition of manganese offers a further advantage in that the reduced iron solubility decreases the rate of attack on the rigging.

I claim:

l. in the method for hot-dip aluminizing of ferrous substrates to produce a protective coating thereon. which comprises; cleaning said substrate and thereafter passing said substrate through a bath of molten aluminum consisting essentially of from about 4 to 11% Si, from about 0.1 to about 3.0% Fe,

the improvement which comprises, adding Mn to said molten bath in an amount sufficient to maintain a concentration of from about 0.2 to about 1.5 percent.

2. The method of claim 1, wherein the bath is maintained at a temperature of from about 1,1 F to 1,300F and the Mn content of the bath is within the range of from about 0.4 to 0.8 percent.

3. The method of claim 2, wherein said bath temperature is about 1,200F to 1,250F and the Fe content is less than about 1.5 percent.

4. The method of claim 2, wherein the Si content of said bath ranges from about 7 to about 9 percent.

. 5. The method of claim 4, wherein the final thickness of said protective coating ranges from about 0.4 to about 2.0 mils; said bath is maintained at a temperature of about 1,200F to 1,250F and the Fe content is less than about l.5 percent.

a; i a: 

1. IN THE METHOD FOR HOT-DIP ALUMINIZING OF FERROUS SUBSTRATES TO PRODUCE A PROTECTIVE COATING THEREON, WHICH COMPRIES, CLEANING SAID SUBSTRATE AND THEREAFTER PASSING SAID SUBSTRATE THROUGH A BATH OF MOLTEN ALUMINUM CONSISTING ESSENTIALLY OF FROM ABOUT 4 TO 11% SI, FROM ABOUT 0.1 TO ABOUT 3.0% FE, THE IMPROVEMENT WHICH COMPRISES, ADDING MN TO SAID MOLTEN BATH IN AN AMOUNT SUFFICIENT TO MAINTAIN A CONCENTRATION OF FROM ABOUT 0.2 TO ABOUT 1.5 PERCENT.
 2. The method of claim 1, wherein the bath is maintained at a temperature of from about 1,180*F to 1,300*F and the Mn content of the bath is within the range of from about 0.4 to 0.8 percent.
 3. The method of claim 2, wherein said bath temperature is about 1,200*F to 1,250*F and the Fe content is less than about 1.5 percent.
 4. The method of claim 2, wherein the Si content of said bath ranges from about 7 to about 9 percent.
 5. The method of claim 4, wherein the final thickness of said protective coating ranges from about 0.4 to about 2.0 mils; said bath is maintained at a temperature of about 1,200*F to 1,250*F and the Fe content is less than about 1.5 percent. 